A Digital Subscriber Line Access Multiplexer (DSLAM) is a network device that is capable of receiving upstream signals from multiple subscribers (e.g., Customer Premises Equipment or CPE) and aggregating these upstream signals onto backbone switching equipment such as a Broadband Remote Access Server (BRAS) of a Network Service Provider (NSP). In a typical CPE/DSLAM/BRAS layout, the communications between the CPE and DSLAM is ATM-based (e.g., Point-to-Point over asynchronous transfer mode or PPPoA) while the communications between the DSLAM and the BRAS is generally packet-based.
A number of standards are available which enable encapsulation of Local Area Network (LAN) and Wide Area Network (WAN) protocols over ATM (e.g., AAL5, AAL3, RBE, PPPoE/PPPoA, IPoA, IMA, and so on). Such standards provide different alternatives for integrating ATM communications into existing WAN and/or LAN communications.
Moreover, since packets can be quite large (e.g., 8,000 bytes in length) but ATM cells are fixed at 53 bytes in length (i.e., the first 5 bytes being header information and the remaining 48 bytes being reserved for data), the overhead cost of using ATM is relatively high. That is, relatively speaking, the 5-byte header is an excessive overhead cost and cuts into the amount of data that can be transferred between the CPE and the DSLAM. Along these lines, the term “ATM cell tax” is often used to describe the overhead cost imposed by ATM cells.
Nevertheless, providers typically offer Quality of Service (QoS) support to their subscribers, and thus intelligently apportion data within the fixed 48-byte data fields of ATM cells in a hands-on manner. One way to accomplish this hands-on apportionment is for the providers to manually configure BRAS equipment to control the downstream flow of network traffic with knowledge that packetized communications between the BRAS and the DSLAM will then face ATM encapsulation from the DSLAM to the CPE. In particular, technicians of the providers manually enter knowledge of the downstream protocols in use from the DSLAM to the CPE (e.g., PPPoA) and a conversion scheme which appropriately accounts for the ATM cell tax. Such operation involves the technicians entering such information through a command line interface (CLI) of the BRAS equipment.
For example, if the BRAS receives a 64-byte packet en route to the DSLAM and if the BRAS knows that the DSLAM will split the 64-byte packet into two 53-byte ATM cells, the BRAS can impose a level of flow control which is commensurate for two 53-byte ATM cells (i.e., 106 bytes in total). Accordingly, the BRAS may impose a scheme for shaping and policing the downstream flow which is more favorable to the DSLAM. As a result, the operation of the DSLAM can be improved (e.g., less dropped packets, enhanced ability to preserve packet priorities, etc.). It should be understood that different encapsulations will have different overhead and thus require different computations.